[ThinLTO] Avoid creating map entries on lookup (NFCI) (#164873)

We could inadvertently create new entries in the PrevailingModuleForGUID
map during lookup, which was always using operator[]. In most cases we
will have one for external symbols, but not in cases where the
prevailing copy is in a native object. Or if this happened to be looked
up for a local.

Make the map private and create and use accessors.

Author: teresajohnson

llvm/include/llvm/LTO/LTO.h

@@ -462,6 +462,19 @@ class LTO {
     ModuleMapType ModuleMap;
     // The bitcode modules to compile, if specified by the LTO Config.
     std::optional<ModuleMapType> ModulesToCompile;
+
+    void setPrevailingModuleForGUID(GlobalValue::GUID GUID, StringRef Module) {
+      PrevailingModuleForGUID[GUID] = Module;
+    }
+    bool isPrevailingModuleForGUID(GlobalValue::GUID GUID,
+                                   StringRef Module) const {
+      auto It = PrevailingModuleForGUID.find(GUID);
+      return It != PrevailingModuleForGUID.end() && It->second == Module;
+    }
+
+  private:
+    // Make this private so all accesses must go through above accessor methods
+    // to avoid inadvertently creating new entries on lookups.
     DenseMap<GlobalValue::GUID, StringRef> PrevailingModuleForGUID;
   } ThinLTO;
 

llvm/lib/LTO/LTO.cpp

@@ -1086,15 +1086,15 @@ LTO::addThinLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
           GlobalValue::getGlobalIdentifier(Sym.getIRName(),
                                            GlobalValue::ExternalLinkage, ""));
       if (R.Prevailing)
-        ThinLTO.PrevailingModuleForGUID[GUID] = BM.getModuleIdentifier();
+        ThinLTO.setPrevailingModuleForGUID(GUID, BM.getModuleIdentifier());
     }
   }
 
   if (Error Err =
           BM.readSummary(ThinLTO.CombinedIndex, BM.getModuleIdentifier(),
                          [&](GlobalValue::GUID GUID) {
-                           return ThinLTO.PrevailingModuleForGUID[GUID] ==
-                                  BM.getModuleIdentifier();
+                           return ThinLTO.isPrevailingModuleForGUID(
+                               GUID, BM.getModuleIdentifier());
                          }))
     return Err;
   LLVM_DEBUG(dbgs() << "Module " << BM.getModuleIdentifier() << "\n");
@@ -1108,8 +1108,8 @@ LTO::addThinLTO(BitcodeModule BM, ArrayRef<InputFile::Symbol> Syms,
           GlobalValue::getGlobalIdentifier(Sym.getIRName(),
                                            GlobalValue::ExternalLinkage, ""));
       if (R.Prevailing) {
-        assert(ThinLTO.PrevailingModuleForGUID[GUID] ==
-               BM.getModuleIdentifier());
+        assert(
+            ThinLTO.isPrevailingModuleForGUID(GUID, BM.getModuleIdentifier()));
 
         // For linker redefined symbols (via --wrap or --defsym) we want to
         // switch the linkage to `weak` to prevent IPOs from happening.
@@ -1988,7 +1988,7 @@ Error LTO::runThinLTO(AddStreamFn AddStream, FileCache Cache,
                                LocalWPDTargetsMap);
 
   auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
-    return ThinLTO.PrevailingModuleForGUID[GUID] == S->modulePath();
+    return ThinLTO.isPrevailingModuleForGUID(GUID, S->modulePath());
   };
   if (EnableMemProfContextDisambiguation) {
     MemProfContextDisambiguation ContextDisambiguation;